Fragmented sleep may impair memory, learning

Forgotten how to do something you just learned yesterday? Consider the possibility that last night's sleep was punctuated by mini-awakenings, robbing you of the ability to commit that new skill to memory. You might have gotten eight hours of sleep, and may not even feel tired. But when sleep is interrupted frequently--as it is in a wide range of disorders, including sleep apnea, alcoholism and Alzheimer's disease--the ability to learn new things can be dramatically impaired, says a new study conducted on mice.

The research, published this week in the Proceedings of the National Academy of Sciences, used a novel method to isolate the effects of sleep fragmentation from overall sleep quality. Studies to date have shown that when sleep is frequently interrupted, memory suffers. But no one really knew whether the memory problems they observed were the result of shorter cumulative sleep times, poor overall sleep quality, the degradation of some distinct part of the sleep cycle, or the sheer annoyance of being prodded awake repeatedly while sleeping. This study suggests that even when frequent waking doesn't affect sleep quality and doesn't cut into overall sleep time, memory takes a hit.

Researchers at Stanford University stimulated "microarousal events" in mice by injecting their brains with a virus carrying a red fluorescent protein. Once established in the brain, the protein found its way to specialized brain cells in the hypothalamus involved in awakening. When stimulated by a laser diode directed at that region of the brain, those specialized neurons became active and the mice briefly awakened. During four hours of daytime sleep, scientists "lit up" the awakening neurons every 60 seconds, causing the mouse's brain briefly to stir, and then fall back to sleep. The frequent awakenings did not drive down the amount of rapid-eye-movement (REM), or deep, restorative sleep the mouse had. Nor did it drive down cumulative sleep time. And it didn't appear to cause the mouse any stress.

A control group of mice had the laser diode flashed at them. But since they had not had the specialized protein introduced into their brain, they did not experience microarousal events, and slept through.

Before tuck-in time, each mouse had been put into a cage where it had two novel areas to explore (when introduced to two new things, a mouse will typically explore both equally). After four hours of daytime sleep, researchers sought to test whether a mouse would remember having explored these areas before.

After four hours of sleep, the mice whose brains had been prodded awake every 60 seconds showed no familiarity with the cage to which they had been introduced earlier, and their patterns of exploration reflected that. The mice whose sleep had not been interrupted behaved as if they remembered having explored the cage.

Researchers suggest that new skills and information are commited to memory--or "consolidated"--during sleep when our brains "replay" recently learned actions or sequences. In the process, the memory, now neatly packaged and ready for storage, is transferred from the hippocampus to the neocortex, from which it can be retrieved when needed. The Stanford researchers who led the study-- biologist H. Craig Heller and neuroscientist Luis de Lecea-- suggested that when frequent awakenings interrupt that process, the memory can be lost or compromised before it is stored.